The central goal of this application is to develop a rational mechanistic basis for employing agents that disrupt the mitogen activated protein kinase (MAPK) signal transduction pathway in order to potentiate the anti-tumor activity of ionizing radiation. This strategy is based upon recent evidence indicating that specific inhibitors of the MAPK pathway interact synergistically with radiation to initiate the apoptotic protease cascade. Our underlying hypothesis is that inhibition of MAPK, an enzyme implicated in G2/M progression, leads to a potentiation of radiation-mediated cell death. We postulate that this phenomenon stems from, or is profoundly influenced by, perturbations in cell cycle regulation, a diminished threshold for tolerating mitochondria) dysfunction, and a reduced ability to survive radiation-induced DNA damage. We have shown in human carcinoma cells that are autocrine regulated via an ErbB 1 and TGFalpha interaction, survive irradiation in a MAPK- dependent fashion. Radiation causes activation of the MAPK pathway and blunting of MAPK activation enhanced the proportion of cells found in G2/M phase 24h after irradiation, which was associated with increased apoptosis. In this proposal we will examine how the MAPK signaling pathway is responsible for cell cycle control, caspase regulation and survival, with measurements of apoptosis and cell cycle regulatory protein expression in carcinoma cells. In Aims 1 and 2 we will test the hypothesis that MAPK inhibition enhances radiation-induced cell killing by potentiating the activation of caspases 8, 9 and 3. In Aim 3 we will test the hypothesis that reduced MAPK activity modifies the radiation / ATM / cdc 2 interaction, altering cell cycle progression leading to enhanced apoptosis. We propose that MAPK signaling is an essential survival factor in the response of the cell to DNA damage. Collectively, these studies will determine the molecular mechanisms by which inhibition of MAPK enhances apoptosis and radio-sensitivity.